My chest tends to feel like it shrunk in size & i hv difficulty gettin air in. i've had xrays + breathing checks, no resp problems.could it be heart?

Possibilities....: There are many different problems that can cause shortness of breath. Have you had pulmonary function testing? This can show whether you have obstruction or restriction which can help narrow things down. Heart problems can also cause shortness of breath. Have you had your oxygen level tested both at rest and with activity? See a lung doctor for a complete evaluation of your shortness of breath.